It is known that an SCR system is an aftertreatment apparatus designed to reduce the nitrogen oxides (NOx) emitted by the internal combustion engine. The SCR system includes a catalyst, which is disposed in an exhaust pipe of the engine to convert the nitrogen oxides into diatonic nitrogen and water with the aid of a reducing agent. The reducing agent may be injected in liquid state into the exhaust pipe by a dedicated injector that is disposed upstream of the catalyst. The injector receives the reducing agent from a tank by a pump which is in fluid communication with the injector via a supply conduit.
The SCR system may also include a pressure sensor for measuring the pressure of the reducing agent within the supply conduit and a number of electric heaters for heating the reducing agent in order to prevent it from freezing under cold environmental conditions.
The SCR system is associated to an electronic control unit (ECU), which is disposed aboard the motor vehicle to carry out on-board diagnostics of the engine. With regard to the SCR system, the ECU is usually provided for testing the proper functioning of the pressure sensor, of the pump, of the injector, of the electric heaters and of the supply conduit. If one of these functionality tests identifies a malfunctioning of the related component, the ECU activates a warning procedure, which provides for signaling to the driver that the motor vehicle needs a maintenance service and which may also limit the engine performance.
Once the maintenance service has been carried out, the warning procedure is only deactivated when the ECU repeats the functionality test of the repaired component, thereby recognizing that the malfunctioning is no longer present. However, each functionality test is only performed under predetermined operating conditions of the internal combustion engine, on that the warning procedure may remain active for a relatively long time after the maintenance service, without providing any immediate evidences that the maintenance service has been successful.